Arcyd: fix fail to push non-ff unknown user branch

Fix the case where a branch is in bad_pre_rev and the branch history is
re-written, we should force push the tracking branch or it will fail.

Test Plan:
Add non-ff update to 'test_unknown_user' in acceptance tests
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
abdt_reporeporter: better unknown user message

When an email address unknown to Phabricator is the primary author of a
review branch, display a more helpful message on the repo dashboard.

Test Plan:
$ ./precommit.sh
read through error message on
$ ./testbed/arcyd/status_html_testbed.sh
Merge branch 'master' into feature/repoconfig